Careers In Earth Science

Water Freedom System

Survive Global Water Shortages

Get Instant Access

Hydrogeologist Earth scientists who map groundwater are called hydrogeologists. They use field methods, maps, and aerial photographs to determine where ground-water is located. To learn more about Earth science careers, visit glencoe.com.

Overuse Groundwater supplies can be depleted. If groundwater is pumped out at a rate greater than the recharge rate, the groundwater supply will decrease and the water table will drop. This is happening to the Ogallala Aquifer. Its water, used mostly for irrigation, is being withdrawn at a rate much higher than the recharge rate.

Subsidence Another problem caused by the excessive withdrawal of groundwater is ground subsidence—the sinking of land. The volume of water underground helps support the weight of the soil, sediment, and rock above. When the height of the water table drops, the weight of the overlying material is increasingly transferred to the aquifer's mineral grains, which then squeeze together more tightly. As a result, the land surface above the aquifer sinks.

A dramatic example of subsidence can be seen along parts of the Gulf coast of Texas, where heavy usage of groundwater over many decades resulted in a wide-scale drop in the ground level. In a region of 12,000 km2, the average subsidence was 15 cm, while some areas dropped by as much as 3 m. This has presented flooding hazards for much of the coastal region.

Pollution in groundwater In general, the most easily polluted groundwater reservoirs are water-table aquifers, which lack a confining layer above them. Confined aquifers are affected less frequently by local pollution because they are protected by impermeable barriers. When the recharge areas of confined aquifers are polluted, however, those aquifers can also become contaminated.

^p Reading Check Identify which kind of aquifer is more vulnerable to pollution.

Sources of groundwater pollution include sewage from faulty septic tanks and farms, landfills, and other waste disposal sites. Pollutants usually enter the ground above the water table, but they eventually infiltrate to the water table. In highly permeable aquifers, pollutants can spread quickly in a specific direction, such as toward the wells shown in Figure 10.15.

Figure 10.15 Pollutants can spread rapidly through a highly permeable aquifer. Note how the polluted well has drawn the pollution toward it as it has withdrawn water from the water table.

Unpolluted well

Water table

Unpolluted well

Water table

Figure 10.15 Pollutants can spread rapidly through a highly permeable aquifer. Note how the polluted well has drawn the pollution toward it as it has withdrawn water from the water table.

Earth Science Careers
Polluted well

Chemicals Because chemicals dissolved and transported with groundwater are submicroscopic in size, they can travel through the smallest pores of fine-grained sediment. For this reason, chemicals such as arsenic can contaminate any type of aquifer. The chemicals generally move downslope from a source in the form of a pollution plume, which is a mass of contaminants that spreads through the aquifer. Once chemical contaminants have entered groundwater, they cannot be easily removed. In the GeoLab at the end of the chapter, you will learn more about how geologists predict the risks of chemical contamination of groundwater based on a region's topography.

^p Reading Check Explain why chemicals such as arsenic can contaminate any kind of aquifer.

Sewage, landfills, and other waste disposal sites can include a variety of contaminants. Chemical contaminants can be leached, meaning dissolved by infiltrating groundwater. When chemical and biological contaminants enter the groundwater, they flow through the aquifer at the same rate as the rest of the groundwater. Over time, an entire aquifer can become contaminated and toxic to humans. Aquifers are particularly vulnerable to pollution in humid areas where the water table is shallow and can more easily come in contact with waste.

Salt Not all pollutants are toxic or unhealthful in and of themselves. For example, ordinary table salt is used to season food, but water is undrinkable when its salt content is too high. In like manner, ground-water is unusable after the intrusion of salt water. Salt pollution is one of the major threats to groundwater supplies, especially in coastal areas, where the intrusion of salt water into groundwater is a major problem. In coastal areas, salty seawater, which is denser than freshwater, underlies the groundwater near Earth's surface, as shown in Figure 10.16. The overpumping of wells can cause the underlying salt water to rise into the wells and contaminate the freshwater aquifer.

Vocabulary

Academic vocabulary

Transport to move from one place to another Airplanes transport packages across the country

Figure 10.16 Freshwater aquifers can become contaminated with salt water. Identify how overpumping can cause the underlying salt water to rise in wells.

Before pumping ouiKB^mMcm^

Interactive Figure To see an animation of salt water contamination, visit glencoe.com.

After pumping

Figure 10.16 Freshwater aquifers can become contaminated with salt water. Identify how overpumping can cause the underlying salt water to rise in wells.

Before pumping ouiKB^mMcm^

Interactive Figure To see an animation of salt water contamination, visit glencoe.com.

After pumping

Salt Water Table Overpumping

Water table Fresh groundwater

Salt water

Salt water

Water table Fresh groundwater

Salt water

Salt water

Table 10.2

Groundwater Pollution Sources

Infiltration from fertilizers

Leaks from storage tanks

Drainage of acid from mines

Seepage from faulty septic tanks

Saltwater intrusion into aquifers near shorelines

Leaks from waste disposal sites Radon

Radon Another source of natural groundwater pollution is radon gas, which is one of the leading causes of cancer in the United States. Radon found in groundwater is one of the products of the radioactive decay of uranium in rocks and sediment, and it usually occurs in very low concentrations in all groundwater. However, some rocks, especially granite and shale, contain more uranium than others. Therefore the groundwater in areas where these rocks are present contains higher levels of radon. Some radon can seep into houses, and, because it is heavier than air, it can accumulate in poorly ventilated basements. The United States Environmental Protection Agency (EPA) advises homeowners in radon-prone regions to have their homes tested regularly for radon gas.

Protecting Our Water Supply

There are a number of ways by which groundwater resources can be protected and restored. First, major pollution sources, many of which are listed in Table 10.2, need to be identified and eliminated. Pollution plumes that already exist can be monitored with observation wells and other techniques. Most pollution plumes spread slowly providing adequate time for alternate water supplies to be found. In some cases, pollution plumes can be stopped by building impermeable underground barriers around the polluted area. Sometimes, polluted groundwater can be pumped out for chemical treatment on the surface.

While these measures can have limited success, they alone cannot save Earth's water supply. Humans must be aware of how their activities impact the groundwater system so that they can protect the water supply. ^

Was this article helpful?

0 0
Growing Soilless

Growing Soilless

This is an easy-to-follow, step-by-step guide to growing organic, healthy vegetable, herbs and house plants without soil. Clearly illustrated with black and white line drawings, the book covers every aspect of home hydroponic gardening.

Get My Free Ebook


Post a comment